1. Field of the Invention
The present invention relates to an X-ray television diagnostic apparatus, and more particularly to a diagnostic apparatus for utilizing a fluoroscopy system in which pulsatory X-rays are intermittently projected toward an object under investigation so as to produce X-ray transmission images, the X-ray transmission images of the object are converted by an image intensifier into optical images, the optical images are then converted by a television camera into electric image signals, and thereafter a plurality of electric image signals are processed by the digital time subtraction technique so as to obtain subtracted images of the object, e.g., angiographic images.
2. Description of the Prior Art
Recently "digital radiography" or "digital fluoroscopy" in which a time subtraction and digital processing are utilized has been applied to an X-ray television diagnostic apparatus such that diagnosis utilizing X-rays is performed for circulatory organs, especially an angiographic examination.
The conventional digital fluorography will now be summarized.
First, an X-ray contrast medium is administrated into a predetermined vein of the arm of a patient as an object under investigation. When a given time "TO" has passed after an injection of the X-ray contrast medium, a diagnostic portion of the vein of the patient into which the X-ray contrast medium has not yet been administered, is photographed by an X-ray television camera device at a first X-ray projection. In general, the X-ray television camera device is constructed by an image intensifier, a television camera and an optical system that is optically coupled between the image intensifier and the television camera. An X-ray television signal output from the television camera device is analogue-to-digital converted into a corresponding digital signal. This digital video signal is stored as a mask image data signal in a first digital memory.
When a given time "T1" has passed after the mask image data signal is stored and the X-ray contrast medium is administered, a second X-ray projection is carried out. Subsequently successive X-ray projections are intermittently done at a given timing ratio, e.g., several projections per one minute, so that the diagnostic portion of the patient into which the X-ray contrast medium is flowing, is photographed by the camera device in accordance with a predetermined time lapse. As a result, a series of digital X-ray video signals is stored in turn in a second digital memory as contrast image data signal. Then time subtraction is performed between the mask image data signal in the first memory and the contrast image data signal in the second memory so as to obtain a digital time subtraction image of the object.
The resultant subtraction image signal in digital form is converted by an A/D converter so as to obtain an analogue subtraction image signal, whereby it is displayed on a monitor screen, or converted by a multi-format camera to obtain a film image. On the other hand, the digital filtering process is executed by utilizing a plurality of X-ray television signals with respect to a time lapse, so that a differential image, low-pass-filtered or high-pass-filtered image is displayed on the monitor screen.
In such a diagnostic apparatus as here set forth, pulsatory X-rays are employed in order to reduce the total quantity of the X-ray projection received by the object. On the other hand, there are the following problems due to afterimage phenomenon on the image intensifier. That is, when an X-ray control pulse is generated synchronized with the field sync pulse of the television camera, an optical (fluorescent) image is output from the image intensifier. This optical image has a waveform in that its leading edge raises sharply, and its trailing edge falls gradually in phase with the trailing edge of the X-ray control pulse. If the scanning operation of the television camera commences immediately after the trailing edge of the X-ray control pulse, a fluctuation of the brightness may occure in the upper and lower regions of the monitor screen because the leading edge of the optical image functions as the afterimage.
To mitigate a above-mentioned drawbacks, the known apparatus has been proposed in, e.g., Japanese Patent Publication No. 56-15184(1981). According to this known apparatus, the blanking signal is applied to the television camera during a time period from the leading edge of the optical image to the disappearance thereof. In other words, during a period defined by adding one sync field period to the afterimage period, the scanning operation of the television camera is blanked out. After such a long blanking period, the scanning operation commences, so as to obtain an X-ray television signal.
There is, however, the other problem that the camera blanking period is susceptible to both the duration time of the X-ray control pulse and the characteristics of the image intensifier. Namely it may not be precisely determined. If the disappearance of this afterimage phenomenon takes a long time (involving several field sync pulse signals), it is therefore required to set a long blanking period. As a result, the scanning operation of the television camera must be delayed for a relatively long time after the X-ray projection is accomplished, so that due to such a long time interval between the successive X-ray projections, available frame numbers per unity time are reduced. Accordingly the resultant image signals cannot precisely indicate the portion under examination having a fast movement.
Moreover since the blanking period of the television camera cannot be assigned to a given period and thus the various timing signals for the write/read signals, scanning of the television camera, and the X-ray projection control signal do not have constant values, there exist drawbacks that the synchronization circuit becomes complex and the production cost of the entire system is expensive.
It is therefore an object of the present invention to provide an X-ray television diagnostic apparatus in which precise X-ray transmission images are available without the adverse influence by the inherent afterimage phenomenon of the image intensifier.
It is another object to provide an X-ray television diagnostic apparatus in which the scanning timing of X-ray transmission images through the image intensifier by the television camera can be precisely determined in synchronization with the field sync pulse signal, and also the X-ray projection intervals can be shortened.